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Glossary Sonar Terms
Sensitivity: Sensitivity is a user feature that adjusts the sensitivity of the sonar system to show more or less detail in
the water. Higher sensitivities are often preferred, however, when the water contains debris (silt, storm debris, etc.) and
it can be difficult to pick out targets. Conversely, if sensitivity is set too low, relevant targets may be missed.
SONAR: SONAR is the acronym for SOund and NAvigation Ranging. Sonar technology uses precision sound bursts
transmitted underwater to determine the distance and other attributes of objects in the water. Distance can be
determined because the speed of sound in water is constant, and the time for the signal to return is measured. Sound
also travels very quickly underwater, making sonar a responsive, cost-effective tool. Sonar is the basic technology
behind all recreational and commercial fishfinding and depthfinding devices.
Sonar Echo Enhancement: Sonar Echo Enhancement is a Humminbird® feature that describes the high degree of sonar
sensitivity achieved through a combination of transmitter/receiver and software algorithms. The result of Sonar Echo
Enhancement is to display virtually everything in the water that is of interest to the angler, including bait fish, game
fish, thermoclines, weed beds, subtle structure, and more.
Sonar Update Rate: Sonar Update Rate is the number of times per second that the transmitter/receiver sends and
receives sonar signals. A very fast sonar update rate collects more information and provides a more detailed image of
the bottom, fish and structure. Many Humminbird® units operate at up to 40 times per second when in single frequency
operation. Due to the limitation of the speed of sound in water, the update rate begins to slow as depth increases to
deeper than 50 feet. In very shallow water (less than 10 feet), however, update rates as much as 60 times per second
can be achieved.
Speed: Speed is the rate at which the boat moves through the water. Boat speed can be measured as Speed Over Ground
or Speed Through Water. Speed Over Ground is provided by GPS, and is the measurement of the boats progress across a
given distance. Speed Through Water is provided by a speed paddlewheel, and is the measurement of the flow past the
boat, which may vary depending on current speed and direction. Speed Through Water is most critical for anglers using
downriggers, as it impacts the running depth of the down riggers. Speed Over Ground is optimal for navigation, as accurate
destination times can be derived from this measurement. Humminbird® products allow for input and display of both
sources.
Structure: Structure is a general term for objects on the bottom that present a discontinuity and are a likely attractor
for fish. This includes bottom contour features (drop-offs, humps, and holes), standing structure (stumps, timbers, brush
piles) and a wide range of other potential objects (sunken boats, reefs). Humminbird® units excel at showing structure
with great detail over a wider area due to unique sonar configurations developed for the angler.
Structure ID®: Structure ID® is a Humminbird® feature that describes the traditional grayscale method of presenting
sonar information. See Grayscale.
Surface Clutter: Surface Clutter is a phenomenon where sonar returns are reflected off of tiny objects near the surface
of the water, including algae and even air bubbles. Typically, saltwater environments have significantly greater surface
clutter than freshwater due to continuous wind and wave action that causes aeration at the surface. The Surface Clutter
menu provides manual control to bias the default settings under extreme conditions.
Target Separation: Target Separation is the measurement of minimum distance that a fishfinder needs to be able to
recognize two very close objects as two distinct targets (i.e. two fish hanging very close, or a fish hanging very close to
structure). Humminbird® fishfinders provide a very good 2 1/2 inches of target separation in shallower than 100 feet of
depth. Target separation decreases as depth increases due to the need for longer Pulse Width to achieve greater depth.
See Pulse Width.
Thermoclines: Thermoclines are water layer(s) of distinctly different temperatures that create a sonar reflection due to
the density of the differing water temperatures. Typically a thermocline will appear as a continuous band across the
display at some distance above the bottom contour. Thermoclines are of interest to anglers because fish will suspend
above or below the thermocline as they seek the optimum temperature and oxygen levels.
Time Variable Gain: Time Variable Gain is a processing step applied to the sonar return to "normalize" the data so that
objects of equal size (i.e. fish) appear to be the same size, even if they are separated by a good distance. Time Variable
Gain is a fundamental attribute of good sonar, but is often promoted as a feature.